1. Technical Field
The present invention relates to optical communications, and more particularly to mitigation of fiber nonlinearities in multilevel coded-modulation schemes.
2. Description of the Related Art
In order to adapt to the ever-increasing demands of telecommunication needs, network operators already consider 100 Gb/s per dense wavelength division multiplexing (DWDM) channel transmission. At those data rates, the performance of fiber-optic communication systems is degraded significantly due to intra- and inter-channel fiber nonlinearities, polarization-mode dispersion (PMD), and chromatic dispersion. To deal with those channel impairments, novel advanced techniques in modulation and detection as well as coding and signal processing should be developed. To deal with chromatic dispersion and PMD, a number of channel equalization techniques have been proposed recently including a digital filtering approach, maximum likelihood sequence detection (MLSD), and turbo equalization. To simultaneously suppress chromatic dispersion and PMD, coded orthogonal frequency division multiplexing (OFDM) has been proposed. On the other hand, to deal with intra-channel nonlinearities, either constrained coding or turbo equalization may be used. Moreover, it has been shown that fiber nonlinearities such as chromatic dispersion and PMD can be simultaneously compensated for by using low-density parity-check (LDPC)-coded turbo equalization. However, the LDPC-coded turbo equalization scheme, although an excellent candidate to mitigate nonlinear and linear impairments simultaneously, so far has been studied for binary transmission only with direct detection.